1. Field of the Invention
The present invention relates to a photodetector for weak light, and more specifically a photodetector for discriminating the number of photons present in an optical pulse in a 1.5 μm band.
2. Description of the Related Art
It is expected that optical communication and information processing technologies taking advantage of quantum properties such as quantum cryptography and quantum teleportation will further advance in the future. These technologies are expected to be introduced into the infrastructure of information and telecommunication technology. Considering the present affinity of quantum communication experiments for techniques for generating, controlling, and detecting a photon correlation pair using a light in a 1.5 μm band that is a fiber low loss wavelength band will be important for future communication. Detection of a single photon is a basic technique for such communication technologies. A photodetector for weak light capable of detecting a single photon is a device which is not only necessary for such communication technologies but also important for inspection of quantum optics phenomena other than communication purposes.
At present, the device which detects a light in the 1.5 μm band is a PIN photodiode or an avalanche photodiode mainly containing InGaAs. The detector used in actual experiments for quantum cryptographic key distribution is an APD in Geiger mode.
Required performances for the 1.5 μm band photodetector in a future quantum communication field, include: high quantum efficiency (80 percents or more), low error rate, high response rate, and capability for discriminating the number of photons. If the APD is used as the detector, it is difficult to satisfy these requirements. The reason is as follows. Since the APD in Geiger mode is intended to improve the sensitivity of the detector, the APD loses information on the number of photons at the peak value of the detector, and thus cannot discriminate the number of photons such as one photon or two photons. In addition, if the sensitivity of the APD is improved, the error rate is increased, accordingly. For example, if a dark count of 0.1 percent is allowed, the quantum efficiency of the APD in Geiger mode is about 32 percents.
In recent years, a photodetector capable of measuring the number of photons of light in a 1.5 μm band using a bolometer made of a superconductor has been developed (see Non-Patent Literature of A. J. Miller, S. Woo, J. M. Martinis, and A. V. Sergienko: “QCMC'02 (2002)”). This photodetector is required to be cooled down to 100 millikelvins (100 mK). In principle, the photodetector does not operate if it is not cooled to a deep cryogenic temperature. Therefore, it is considered difficult to utilize this photodetector for communication purposes. In addition, the photodetector has disadvantages such as low coupling efficiency with respect to a fiber, and low quantum efficiency, evaluated at 20 percent.